The present invention relates to an under voltage lockout for switching mode power supplies and more particularly to a lockout circuit which greatly reduces initial current drain from an energy storage capacitor until the voltage on that storage capacitor has reached a level to enable complete starting of the power supply and regulator.
In switching mode power supplies, it is common for the power supply to itself provide the power for energizing the associated regulator and control circuitry. As is understood it is therefor necessary to provide some sort of circuit which initiates operation of the system from a separate current source. Typically, rectified line voltage is applied to charge a capacitor which can hold enough energy to sustain the initial operation of the power supply until its output circuit can generate enough power to sustain the regulator and control circuitry. Typically also, the storage capacitor is charged through a resistor from the rectified line voltage. If the regulator and start-up circuit draw significant current from the capacitor during the initial charging, the charging must, in effect, proceed against this load and substantial dissipation can develop in the dropping resistor.
It has heretofore been recognized as advantageous to delay energization of the band-gap regulator typically associated with switching mode power supplies, such an expedient being, for example, disclosed in my earlier U.S. Pat. No. 4,740,742 issued on Apr. 26, 1988 and entitled Voltage Regulator Start-Up Circuit. In such prior art systems, the voltage on the storage capacitor was typically monitored by a comparator which operated to inhibit operation of the band-gap regulator until the voltage on the capacitor exceeded a pre-selected threshold. The same comparator was typically utilized to shut the power supply down if the output voltage dropped below a given level, i.e. in order to protect the power supply circuitry or its load. As is understood by those skilled in the art, the comparator itself necessarily draws some current from the storage capacitor thus slowing its charging or, conversely, requiring a heavier charging current causing more dissipation in the dropping resistor.
While the circuit of the present invention includes a comparator which is responsive to supply voltage, the comparator circuit itself is kept de-energized while the storage capacitor is initially charging. Essentially, the only current taken from the storage capacitor during charging is directed toward energizing a voltage reference which provides a reference voltage for the comparator. Only when the voltage reference is energized is the comparator switched into an operative state.
Among the several objects of the present invention, it may be noted the provision of a novel under voltage lockout circuit for a switching mode power supply; the provision of such a start-up circuit which requires minimal current to charge up a storage capacitor; the provision of such a circuit which substantially reduces power dissipation; the provision of such a circuit which is highly reliable and which is of relatively simple and inexpensive construction. Other objects and features are in part apparent and in part pointed out hereinafter.